1. Field of the Invention
This invention relates to video to film conversion.
2. Description of the Prior Art
Recent developments in video signal processing have enabled a whole host of different video effects and operations, and the use of high definition digital video provides quality which is approaching, or even surpassing that obtainable by conventional film. However, for many years to come, there will be a need to be able to distribute as film or the like material which has been processed, if not also acquired, in high definition digital video format.
One popular video standard (SMPTE 240M) is characterised by 60 fields per second and a 2:1 interlace. However, established standards for film are 30 frames per second and 24 frames per second. Sony Corporation has developed an electron beam recorder which can produce film material from a video signal and has also developed apparatus for converting the video signal from a standard format to a format which the electron beam recorder can use.
In one previously proposed process for recording SMPTE 240M video as 30 frame/s film, called a "field combination process", pairs of temporarily adjacent fields of the 60 field/s video signal are composited to produce frames at 30 frame/s, and the frames are recorded on film. However, this process produces film which suffers from "double-imaging" in areas of motion. Therefore, an improved process has been developed, involving "motion compensated temporal interpolation", in which: (i) frames at 60 frame/s are produced, each either from one or from three of the input fields, (ii) motion is detected in areas of the picture between pairs of temporally adjacent frames, (iii) output frames are produced at 30 frame/s, with each pixel of each output frame derived from pixels in the respective pair of 60 frame/s spatially displaced from the output pixel in dependence upon the detected motion and the temporal offset between the output frame and the two progressive scan frames from which it is formed, and (iv) the output frames are recorded on film. (Interlace to progressive scan conversion is described in detail in United Kingdom patent application GB 2231228A, especially with reference to FIGS. 5 to 14 thereof). Motion compensated temporal interpolation is also described in GB 2231228A, especially with reference to FIGS. 15 to 48 thereof. Conversion from 60 field/s 2:1 interlace video to 30 frame/s film is described in detail in United Kingdom patent application GB 2249907A particularly with reference to FIGS. 57 and 60 thereof. The contents of GB 2231228A and GB 2249907A are incorporated herein by reference as if printed in full below.) This motion compensated temporal interpolation process is computationally intensive, and therefore requires complex, bulky and expensive apparatus, and with present technology cannot operate at real-time rate, but it does overcome to a large extent the double imaging problem.
In another previously proposed process for recording SMPTE 240M video as 24 frame/s film, known as the "drop field process", in a sequence of ten fields of the video signal, fields 3 and 8, for example, are ignored, the fields of each remaining pair of temporally adjacent fields (1 and 2; 4 and 5; 6 and 7; and 9 and 10) are composited to produce four frames at 24 frame/s, and these frames are recorded on film. Like the field combination process, the drop field process also suffers from double imaging in areas of motion. Furthermore, there is a 12 Hz temporal judder because a predetermined pair of the ten input fields are ignored. Therefore, an improved process has been developed, again involving motion compensated temporal interpolation, in which: (i) frames at 60 frame/s are produced, each either from one or from three of the input fields; (ii) motion is detected in areas of the picture between pairs of temporally adjacent frames; (iii) output frames are produced at 24 frame/s, four for every ten of the 60 frame/s frames, with each pixel of each output frame derived from pixels in a respective pair of the 60 frame/s frames spatially displaced from the output pixel in dependence upon the detected motion and the temporal misalignment between the output frame and the pair of frames from which is formed; and (iv) the output frames are recorded on film. Conversion from 60 field/s 2:1 interlace video to 24 frame/s film is described in GB 2231228A with reference to FIGS. 1 to 48 thereof. Again, this form of motion compensated temporal interpolation is computationally intensive, requiring complex, bulky and expensive apparatus, but it does overcome the double imaging problem and the judder problem to a large extent.
The lower output frame rate of 24 Hz can only support motion resulting in temporal frequencies up to 12 Hz in a picture without alias. Motion with components between 12 Hz and 30 Hz can be captured but only as aliased components. When the picture is down converted to the 24 Hz frame rate, even using motion compensated interpolation, all motion with temporal frequency components above 12 Hz will be aliased to lower frequencies and produce subjectively poor motion portrayal. Both the drop field process and the more complex motion compensated interpolation process produce the correct output component rate, but introduce motion artifacts in the output picture due to the down conversion from 30 Hz to 24 Hz frame rate. A similar problem arises in conversion from 60 field/s 2:1 interlace to 30 frame/s film. However, the motion artifacts are less pronounced.
When a picture is shot on a set or on location using video, the video material can immediately be viewed using a video monitor or viewfinder. It is, however, impracticable to convert the material to film, on set or on location, using the processes mentioned above, because the cost and size of the required equipment are prohibitive. When the video material is viewed on set or on location, the above mentioned motion artifacts which may later be introduced by or become apparent due to the video to film conversion process will not be apparent, and if the subsequent conversion process introduces artifacts which are unacceptable, it is extremely expensive to reshoot the video at a later date, for example with slower pans or action, in an attempt to obtain an artifact free end product.